Coccidiosis is a widespread poultry disease which is produced by infections with protozoans of the genus Eimeria which cause severe pathology in the intestines and ceca of poultry. Some of the most significant of these species are E. tenella, E. acervulina, E. necatrix, E. brunetti and E. maxima. This disease is generally spread by the birds picking up the organism at its infectious stage in droppings on contaminated litter or ground, or by way of food or drinking water. The disease is manifested by hemorrhage, accumulation of blood in the ceca, passage of blood in the droppings, weakness and digestive disturbances. The disease often terminates in the death of the animal, but the fowl which survive severe infections have had their market value substantially reduced as a result of the infection. Coccidiosis is, therefore, a disease of great economic importance and extensive work has been done to find new and improved methods for controlling and treating coccidial infections in poultry.
In the poultry industry it is common practice to include anticoccidial agents in poultry feed for most of the bird's life. Because the continuous administration of an anticoccidial agent promotes the likelihood of resistance to the agent, most poultry growers have adopted a so-called "shuttle program." In this medication strategy two or more anticoccidial agents are used sequentially during the broiler growout period. In a typical shuttle program the chickens are fed a first anticoccidial product (starter compound) for 21 days, and then switched to a ration containing a different anticoccidial product (grower compound). There may be a finisher product applied towards the end of the growout, and/or a 5-10 day drug withdrawal period. The principal objective of shuttle programs is to prevent or delay the emergence of drug-resistant coccidial strains that may be selected by continuous medication, as well as to control the disease itself.
The present inventors have found that the sequential use of anticoccidial agents leaves windows of compromised efficacy during or shortly after switching compounds, as manifested in increased lesions and oocyst shedding and decreased anticoccidial indices. This finding accords with industry observations that litter counts in commercial operations peak during the fourth week of the growout period, i.e. a week following the switch from starter to grower products in shuttle programs. Thus, even though the "shuttle" program may slow down resistance development, and/or the disease, it does not represent the optimal medication strategy in view of the gap of efficacy that may result from the switching of drugs.